1. Field of the Invention:
The present invention relates to electrocoating compositions containing a resinous phase dispersed in an aqueous medium, the resinous phase including an ionic electrodepositable resin with improved corrosion resistance formed from a hydrolytically stable ionic polyester polymer.
2. Description of the Related Art
Electrodeposition as a coating application method involves deposition of a film-forming composition onto a conductive substrate under the influence of an applied electrical potential. Electrodeposition has become increasingly important in the coatings industry because, by comparison with non-electrophoretic coating means, electrodeposition offers increased paint utilization, improved corrosion protection and low environmental contamination.
U.S. Pat. No. 5,739,213 and U.S. Pat. No. 5,811,198, both to Freriks et al. disclose certain polyester compositions and glycidyl esters thereof, which are suitable for use in powder coatings. The compositions are resins obtainable by reaction of: an aromatic and/or cycloaliphatic carboxylic acid compound (a), comprising two aromatic and/or secondary aliphatic carboxyl groups or the anhydride thereof; a hydroxyl compound (b) comprising two primary or secondary aliphatic hydroxyl groups; at least one hydroxyl substituted carboxylic acid compound (c) comprising at least one tertiary aliphatic carboxyl group and two primary or secondary aliphatic hydroxyl groups; and optionally, one carboxylic acid compound (d) comprising one carboxyl group. The molar ratio of compounds a:b:c:d is (X+Yxe2x88x921):X:Y:Z, wherein X ranges from 2 to 8, Y ranges from 2 to 8, and Z ranges from 0 to 2. While these compositions are known for their use in powder coatings, the applicability of their use in eletrocoating resins as a main vehicle has not been explored.
Certain polyesters have previously found use in electrodeposition applications as chain extenders and crosslinking agents. For instance, U.S. Pat. No. 4,148,772 discloses chain extension of polyepoxides with polyester polyols. The polyester polyols are formed from a variety of dicarboxylic acids and diols which were known in the art to be suitable chain extenders. However, use of the disclosed higher molecular weight polyester polyols in chain extension is hindered by the overall hydrolytic instability of the ester bonds in aqueous solutions.
It, therefore, would be advantageous to provide an electrocoating composition that combines the superior coating ability, flexibility, durability and corrosion resistance of polyesters with the hydrolytic stability of traditional electrodepositable compositions.
In accordance with the present invention, an electrodepositable coating composition comprising a resinous phase dispersed in an aqueous medium is provided. The resinous phase comprises an ionic polyester polymer. The ionic polyester polymer comprises the reaction products of a) an aromatic and/or cycloaliphatic carboxylic acid compound comprising at least two aromatic and/or secondary aliphatic carboxyl groups, or an anhydride thereof, b) a branched aliphatic, cycloaliphatic or araliphatic compound containing at least two aliphatic hydroxyl groups, the aliphatic hydroxyl groups being either secondary or tertiary hydroxyl groups or primary hydroxyl groups attached to a carbon adjacent to a tertiary or quaternary carbon; c) a compound comprising an ionic salt group or a group which is converted to an ionic salt group; and d) optionally, at least one hydroxyl substituted carboxylic compound comprising at least one tertiary aliphatic carboxyl group and at least two aliphatic hydroxyl groups. The ionic polyester polymer preferably has an ionic salt group equivalent weight of 1,000 to 10,000. Preferably, the carboxyl:hydroxyl equivalent ratio of a to b or of a to (b+d) is greater than 1:1.
Also provided in accordance with the present invention is a method for electrocoating a conductive substrate which serves as an electrode in an electrical circuit the conductive substrate and a counter-electrode. The method comprises the steps of: i) immersing the conductive substrate and the counter-electrode into an aqueous electrocoating composition comprising the resinous phase described above, the ionic polyester polymer of the resinous phase having functional groups which are reactive with a curing agent. The electrocoating composition further includes a curing agent having functional groups reactive with the reactive functional groups of the ionic polyester polymer; and ii) applying a direct current between the conductive substrate and the counter-electrode so as to deposit a film derived from the resinous phase.
Provided, also, is a substrate prepared according to the above-described method.
The electrodepositable coating composition of the present invention includes a resinous phase dispersed in an aqueous medium, the resinous phase includes an ionic polyester polymer comprising the reaction product of the following reactants:
a. an aromatic and/or cycloaliphatic carboxylic acid compound comprising at least two aromatic and/or secondary aliphatic carboxyl groups, or an anhydride thereof;
b. a branched aliphatic, cycloaliphatic or araliphatic compound containing at least two aliphatic hydroxyl groups, said aliphatic hydroxyl groups being either secondary or tertiary hydroxyl groups or primary hydroxyl groups attached to a carbon adjacent to a tertiary or quaternary carbon;
c. a compound comprising an ionic salt group or a group which is converted to an ionic salt group; and
d. optionally, at least one hydroxyl substituted carboxylic compound comprising at least one tertiary aliphatic carboxyl group and at least two aliphatic hydroxyl groups.
The density of ionic groups on the polyester polymer will affect the ability of the polyester polymer to coat the substrate. Depending upon the final structure of the polyester polymer, the compounds included in the resinous phase and in the aqueous medium and the size, shape and metallic composition of the conductive substrate to be coated, the ionic salt group equivalent weight of the ionic polyester polymer will vary. Preferably, the ionic salt group equivalent weight of the polyester polymer is between 1,000 and 10,000.
As described above, the polyester polymer includes compounds (a)-(c), and optionally, (d). Compound (a) is an aromatic and/or cycloaliphatic carboxylic acid compound comprising at least two aromatic and/or secondary aliphatic carboxyl groups, or an anhydride thereof. Compound (a) is, for example, phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid methylhexahydrophthalic acid, terephthalic acid, isophthatic acid, decahydronaphthalene dicarboxylic acid, endomethylene tetrahydrophthalic acid, methylendomethylene tetrahydrophthalic acid, or anhydrides thereof, or combinations thereof.
The polyester polymer is also derived from compound (b), a branched aliphatic, cycloaliphatic or aralaphatic compound containing at least two aliphatic hydroxyl groups. The aliphatic hydroxyl groups are either secondary or tertiary hydroxyl groups or primary hydroxyl groups attached to a carbon adjacent to a tertiary or quaternary carbon. A typical class of branched aliphatic compounds having primary hydroxyl groups attached to a carbon adjacent to a tertiary or quaternary carbon are 2,2-dialkyl-propane-1,3-diols. The two 2-alkyl groups can be the same or different and are typically branched or unbranched C1-C6 alkanes. Compound (b) can also be a cycloalphatic or araliphatic compound, such as, for example, hydrogenated bisphenol A, dihydroxyl cyclohexane, dimethylol cyclohexane, dihydroxybenzene or combinations thereof.
Compound (c) is a compound comprising an ionic salt group or a group that is converted to an ionic salt group. The salt group can confer either an overall positive or negative charge to the ionic polyester polymer. A compound which is xe2x80x9ca compound comprising an ionic salt groupxe2x80x9d is a compound which includes the ionic salt group prior to polymerization with compounds (a) and (b). A xe2x80x9ccompound comprising a group which is converted to an ionic salt groupxe2x80x9d is a compound which, when reacted with another compound, forms a salt group. An example of a compound comprising a group that is converted to an ionic salt group is epichlorohydrin which, when reacted with, a sulfide and lactic acid, forms a sulfonium group with a lactate counterion.
In a preferred embodiment compounds (a) and (b) are reacted in a ratio such that the carboxyl:hydroxyl equivalent ration of (a) to (b) is greater than one. This ensures that the resultant polymer includes at least one free carboxyl group to be further reacted with, for example, compound (c) or a counterion to add a salt group or a chain extending molecule. Most preferably, the carboxyl:hydroxyl equivalent ratio of (a) to (b) is within the range of 1.1 to 2:1. Because compound (d), when included in the ionic polyester polymer includes both a carboxyl and at least two hydroxyl groups, its addition to the reaction of (a) with (b) alters the overall number of reactive carboxyl groups and hydroxyl groups in the reaction, and, therefore, the preferred ratios of compounds (a), (b) and (d). Preferably, the carboxyl:hydroxyl equivalent ratio of (a) to ((b)+(d)) is 0.1 to 2:1. With respect to the relative amounts of compounds (b) and (d), the ratio of hydroxyl equivalents of (b) to (d) is, preferably, 0.1 to 10:1.
In a first embodiment of (c), the ionic salt group is a salt of a carboxylic acid. The dicarboxylic compound (a) used to prepare the polyester polymer includes carboxyl groups. If compound (a) is reacted with compound (b) in a molar excess of carboxyl groups and in the presence of a suitable counterion, compound (a) can be considered as a compound comprising a group which is converted to an ionic salt group.
However, most anionic resins are more complex. For instance, as described above, when compounds (a) and (b) are reacted with a molar excess of carboxyl groups, the resultant polyester includes reactive carboxyl groups. These reactive carboxyl groups can be reacted with, for example, a variety of co-reactive functional groups containing compounds which can include salt groups or groups which can be converted to salt groups. Examples of co-reactive functional groups are hydroxyl and epoxy and examples of co-reactive functional group-containing compounds are hydroxyl carboxylic acids and epihalohydrins.
In addition to use of carboxylic groups to confer a negative charge to the ionic polyester polymer, the negative charge can be conferred via phosphate groups. As above, the polyester polymer is formed from compounds (a) and (b) in a ratio suitable to create an excess of carboxyl groups, which are reacted with an epihalohydrin. The resultant free epoxy group is reacted with phosphoric acid to form anionic groups. This process is disclosed in European Patent Application No. 0 469 491.
As with the anionic salt groups, cationic salt groups can be either present on compound (c) when it is reacted with (a) and (b) or a polymer thereof, or they can be later formed. For electrodeposition, the cationic salt group is typically a quaternary ammonium group, and amine salt group or a sulfonium group. A method for forming quaternary amine groups in a cationic resin is described in U.S. Pat. No. 5,908,912. A method for forming amine salt groups is described in U.S. Pat. No. 4,017,438. Typically, for electrodepositable resin, the cationic salt group is formed by reacting a compound having an epoxy group with a cationic salt group former such as a tertiary amine, a phosphine or a sulfide. For example, a polyester polymer, formed from reacting compounds (a) and (b) in an excess of carboxyl groups, is reacted with an epihalohydrin, such as epichlorohydrin. The resultant polymer has at least one reactive epoxy group. The epoxy groups are then reacted with a sulfide, for example, thiodiethanol, in the presence of lactic acid to form a positively charged sulfonium salt group with a lactate counterion. The thiodiethanol supplies reactive hydroxyl groups which are later reacted with a curing agent. Sulfonium salt groups are preferred primarily due to their resistance to discoloration.
Compound (d) is a hydroxyl substituted carboxylic compound having one tertiary aliphatic carboxyl group and at least two aliphatic hydroxyl groups. Examples of compound (d) are dimethylolyl propionic acid and dihydroxy pivalic acid.
Common to compounds (a), (b) and (d) are that when they are combined into a polyester polymer, they form sterically hindered ester links which, theoretically, stabilize the ester links in an aqueous solution. It is well-known that esters are unstable in water due to the reversibility of the formation of ester groups in the presence of H+. According to the polyesters of the present invention, the reversibility of the formation of ester links is hampered by the local presence of groups which sterically hinder the ester link. Thus, the aromatic or cycloaliphatic carboxyl groups of compounds (a), the secondary or tertiary hydroxyl groups of (b) and the close proximity of a tertiary or quaternary (c) group to the hydroxyl groups of (b) and (d) yield a polyester polymer which is stable in water. These compounds also exhibit superior stability and effective life once cured on the conductive substrate in contrast to typical polyesters.
Suitable polyesters for use as precursor compounds to the ionic polyester polymer of the present invention are described in U.S. Pat. Nos. 5,739,213 and 5,811,198, described above. One example of the polyester precursor compounds is shown below as formula I: 
wherein n=1-5.
A second suitable precursor polyester polymer is shown below as formula II: 
wherein n=1-15.
A third suitable precursor polyester polymer compound has the formula III: 
wherein n=1-15.
The above-described polyester precursor compounds are typically reacted with a sulfide, most preferably thiodiethanol, in the presence of an organic acid, such as boric acid, to form a cationic polyester polymer.
The epoxy-terminal polyester materials described above can be extended to produce higher molecular weight polymer compositions. This is achieved by reacting the epoxy-terminal polyester material with a material having two or more pendant or terminal groups that are reactive with the epoxy groups. These epoxy-reactive groups are typically, for example and without limitation, carboxyl, hydroxyl and amine groups. For example, as show in Example III, below, an epoxy-functional polyester is reacted with the di-acid cyclohexanedicarboxylic acid. The reaction between the epoxy-terminal polyester material and the material having epoxy-reactive pendant or terminal groups is preferably performed with a molar excess of epoxy groups. This results in an extended product having terminal epoxy groups that can be reacted with a suitable ionic group forming compound, such as a sulfide, to form an ionic group, such as a sulfonium group.
The ionic polyester polymer contains at least one functional group which is reactive with a curing agent. Typically, the reactive functional group is an active hydrogen group, as described in U.S. Pat. No. 5,908,912, which is most preferably a hydroxyl group. In the case of cationic embodiments of the resins, a hydroxyl group is present on the polyester polymer as a result of the opening of the epoxy ring during formation of the cationic groups.
Preferably, the ionic polyester includes active hydrogens which are generally reactive with curing agents for transesterification, transamidation, and/or transurethanization with isocyanate and/or polyisocyanate curing agents under coating drying conditions. Suitable drying conditions for at least the partially capped or blocked isocyanate curing agents include elevated temperatures, preferably in the range of 93xc2x0 C. to 204xc2x0 C., most preferably 121xc2x0 C. to 177xc2x0 C., as are known to those skilled in the art. Preferably, the ionic polyester polymer will have an active hydrogen content of 1.7 to 10 milliequivalents, more preferably 2.0 to 5 milliequivalents of active hydrogen per gram of resin solids.
The ionic polyester may also have sterically bulky groups, such as capped isocyanates, grafted thereto in order to improve flexibility of the ionic polyester. These groups can be grafted to the ionic polyester by standard means, such as by reaction of a pendant hydroxyl group of the ionic polyester with a half-capped diisocyanate, i.e., half-capped isophorone diisocyanate.
Typically, the ionic polyester polymer is present in the ED composition in amounts of 55 to 75, preferably 65 to 70 percent by weight based on weight of main vehicle resin solids. By xe2x80x9cmain vehicle resin solids,xe2x80x9d it is meant resin solids attributable to the ionic polyester polymer and the curing agent(s) therefor.
The curing agents for the ED composition of the present invention can be a polyisocyanate curing agent which is preferred for use with cationic polyester polymers and an aminoplast curing agent which is preferred for use with anionic polymers. The polyisocyanate curing agent may be a fully capped polyisocyanate with substantially no free isocyanate groups, such as described in the aforementioned U.S. Pat. No. 4,017,438, or it may be partially capped and reacted with the resin backbone as described in U.S. Pat. No. 3,984,299, U.S. Pat. No. 5,074,979 and U.S. Pat. No. 4,009,133. The polyisocyanate can be an aliphatic or an aromatic polyisocyanate or a mixture of the two. Diisocyanates are preferred, although higher polyisocyanates can be used in place of or in combination with diisocyanates.
Any suitable aliphatic, cycloaliphatic or aromatic alkyl monoalcohol or phenolic compound may be used as a capping agent for the polyisocyanate curing agent, for example, lower aliphatic alcohols such as methanol, ethanol, and n-butanol; cycloaliphatic alcohols such as cyclohexanol; aromatic-alkyl alcohols such as phenyl carbinol and methylphenyl carbinol; and phenolic compounds such as phenol itself and substituted phenols wherein the substituents do not affect coating operations, such as cresol and nitrophenol. Glycol ethers may also be used as capping agents. Suitable glycol ethers include ethylene glycol butyl ether, diethylene glycol butyl ether, ethylene glycol methyl ether and propylene glycol methyl ether. Other suitable capping agents include oximes such as methyl ethyl ketoxime, acetone oxime and cyclohexanone oxime, lactams such as epsilon-caprolactam, and amines such as dibutyl amine.
As described above, the curing agent can be an aminoplast resin. Aminoplast resins are the condensation product of an aldehyde, e.g., formaldehyde, acetaldehyde, crotonaldehyde, and benzaldehyde with an amine- or amido group-containing substance, e.g., urea, melamine, and benzoguanamine. Products obtained from the reaction of alcohols and formaldehyde with melamine, urea or benzoguanamine are preferred in the aqueous-based coating compositions because of their good water dispersibility. Useful alcohols used to make the etherified products are the monohydric alcohols, such as methanol, ethanol, propanol, butanol, hexanol, benzyl alcohol, cyclohexanol, and ethoxyethanol. An etherified melamine-formaldehyde resin is the preferred aminoplast resin. U.S. Pat. No. 4,075,141 to Porter et al. contains a description of useful aminoplast resins and is incorporated herein by reference.
The curing agent is typically present in the ED composition in amounts of 25 to 45, preferably 30 to 35 percent by weight based on weight of main vehicle resin solids.
The ionic electrodepositable resin described above is present in the electrocoating composition in amounts of about 1 to about 60 percent by weight, preferably about 5 to about 25 based on total weight of the electrodeposition bath.
The aqueous compositions of the present invention are in the form of an aqueous dispersion. The term xe2x80x9cdispersionxe2x80x9d is believed to be a two-phase transparent, translucent or opaque resinous system in which the resin is in the dispersed phase and the water is in the continuous phase. The average particle size of the resinous phase is generally less than 1.0 and usually less than 0.5 microns, preferably less than 0.15 micron. The concentration of the resinous phase in the aqueous medium is at least 1 and usually from about 2 to about 60 percent by weight based on total weight of the aqueous medium. When the compositions of the present invention are in the form of resin concentrates, they generally have a resin solids content of about 20 to about 60 percent by weight based on weight of the aqueous medium.
Electrodeposition baths are typically supplied as two components: (1) a clear resin feed, which includes generally the ionic electrodepositable resin, i.e., the main film-forming polymer, and/or crosslinker and any additional water-dispersible, non-pigmented components; and (2) a pigment paste, which generally includes one or more pigments, a water-dispersible grind resin which can be the same or different from the main film-forming polymer, and, optionally, additives such as wetting or dispersing aids. Electrodeposition bath components (1) and (2) are dispersed in an aqueous medium which comprises water and, usually, coalescing solvents. In one embodiment, the grind resin includes a polymer having a sterically bulky group grafted thereto in order to improve pigment wetting and/or flexibility of the grind resin polymer. The grafted group is attached to the polymer by any means known to one skilled in the art. For example, a capped diisocyanate group can be grafted onto a hydroxy-functional polymer by reacting the polymer with a half-capped diisocyanate, such as half-capped isophorone diisocyanate, as shown in Example VI, below.
As aforementioned, besides water, the aqueous medium may contain a coalescing solvent. Useful coalescing solvents include hydrocarbons, alcohols, esters, ethers and ketones. The preferred coalescing solvents include alcohols, polyols and ketones. Specific coalescing solvents include isopropanol, butanol, 2-ethylhexanol, isophorone, 2-methoxypentanone, ethylene and propylene glycol and the monoethyl, monobutyl and monohexyl ethers of ethylene glycol. The amount of coalescing solvent is generally between about 0.01 and 25 percent and when used, preferably from about 0.05 to about 5 percent by weight based on total weight of the aqueous medium.
As discussed above, a pigment composition and, if desired, various additives such as surfactants, wetting agents or catalyst can be included in the dispersion. The pigment composition may be of the conventional type comprising, for example, iron oxides, lead oxides, strontium chromate, carbon black, coal dust, titanium dioxide, talc, barium sulfate, as well as color pigments such as cadmium yellow, cadmium red, chromium yellow and the like. The pigment content of the dispersion is usually expressed as a pigment-to-resin ratio. In the practice of the invention, when pigment is employed, the pigment-to-resin ratio is usually within the range of about 0.02 to 1:1. The other additives mentioned above are usually in the dispersion in amounts of about 0.01 to 3 percent by weight based on weight of resin solids.
When the aqueous dispersions as described above are employed for use in electrodeposition, the aqueous dispersion is placed in contact with an electrically conductive anode and an electrically conductive cathode, with the surface to be coated being the cathode in cationic electrodeposition and the anode in anionic electrodeposition. Following contact with the aqueous dispersion, an adherent film of the coating composition is deposited on the substrate which is serving as an electrode when a sufficient voltage is impressed between the electrodes. The conditions under which electrodeposition is carried out are, in general, similar to those used in electrodeposition of other types of coatings. The applied voltage may be varied and can be, for example, as low as 1 volt to as high as several thousand volts, but typically between 50 and 500 volts. The current density is usually between 0.5 ampere and 5 amperes per square foot and tends to decrease during electrodeposition indicating the formation of an insulating film. The electrodepositable coating compositions of the present invention can be applied to a variety of electroconductive substrates, especially metals such as steel, aluminum, copper, magnesium and conductive carbon coated materials.
After the coating has been applied by electrodeposition, it is cured usually by baking at elevated temperatures such as about 90xc2x0 C. to about 260xc2x0 C. for about 1 to about 40 minutes.